Method and system for control of an EGR valve during lean operation in a boosted engine system

ABSTRACT

Methods and systems are provided for reducing EGR estimation errors during lean engine operating conditions. During lean engine operation, EGR is disabled if the estimated exhaust air-fuel ratio becomes leaner than a lean threshold. The lean threshold is adjusted based on an upper limit of EGR errors that may be tolerated by the engine at a given engine speed and load.

FIELD

The present application relates to control of an EGR valve during leanoperation in a boosted vehicle engine system.

BACKGROUND AND SUMMARY

Engine systems may be configured with exhaust gas recirculation (EGR)systems via which at least a portion of the exhaust gas is recirculatedto the engine intake. Various sensors may be coupled in the enginesystem to estimate the amount of EGR being delivered to the engine.These may include, for example, various temperature, pressure, oxygen,and humidity sensors coupled to the engine intake manifold and/or theexhaust manifold.

As such, EGR levels may be adjusted based on various conditions such ascombustion air-fuel ratio and exhaust emissions levels. One example ofsuch an adjustment is shown by Schilling et al in US 2013/0104544.Therein, during lean operation, an amount of EGR recirculated isincreased so as to improve exhaust NOx emissions.

However, the inventors herein have identified potential issues with suchan approach. As an example, engine control may be degraded due toincorrect EGR estimation. Specifically, during lean to very lean engineoperation, there may be significant amounts of fresh air in the exhaust,and therefore in the EGR. If EGR delivery is estimated by a deltapressure sensor across the EGR valve, or across a fixed orifice of theEGR system (or an EGR MAF sensor), the sensor may incorrectly interpretthe flow of fresh air as exhaust residuals, and EGR may beoverestimated. As a result, adjustments to spark timing, throttleposition, and other actuators based on this EGR estimate may bescheduled incorrectly leading to potential combustion and torque controlissues. As another example, in engine systems where the EGR is estimatedby an intake oxygen sensor, the fresh air may lead to an EGR measurementerror that may be misinterpreted as reduced engine dilution, and EGR maybe underestimated. In addition to incorrect engine control, this mayalso lead to OBD issues due to an EGR monitor noting a discrepancybetween the expected EGR valve flow versus the EGR measured by theintake oxygen sensor. In still other systems, such as MAF systems, thefresh air through the EGR system may be unaccounted for, leading toaircharge calculation errors which may lead to fueling and torqueerrors.

The inventors herein have recognized that during lean operations wherethere is substantial fresh air in the exhaust, it may more efficient tonot deliver any EGR rather than delivering an incorrect amount of EGR.In other words, the benefits of EGR on emissions and fuel efficiency maynot outweigh the fuel penalty and performance penalty incurred due toincorrect EGR estimation and delivery. Thus in one example, some of theabove issues may be at least partly addressed by a method for an enginecomprising: while operating an engine with an air-fuel ratio adjusted tobe leaner than stoichiometry, in response to exhaust air-fuel ratiobeing leaner than a threshold, closing an EGR valve.

As an example, during lean engine operation, a threshold may be setbased on engine speed and load. The threshold may be based on a maximumamount of EGR error that may be tolerated. The tolerable EGR error maybe used to calculate a degree of leanness that is acceptable. Inresponse to the exhaust air-fuel ratio being leaner than the threshold,EGR delivery may be disabled by closing the EGR valve. Herein, the EGRmay be a low pressure EGR and the EGR valve may be an EGR valve coupledin the low pressure EGR system. The valve may be maintained closed atleast until engine operation has returned to be richer than the leanthreshold, such as when engine operation at stoichiometry is resumed.Thereafter, EGR may be enabled.

In this way, issues associated with incorrect EGR estimation andmisdiagnosis of an EGR system by an EGR monitor can be reduced. As such,this not only reduces combustion issues related to incorrect spark andtorque control, but also reduces costs associated with failed EGRmonitors. By removing the conditions that could cause false reading ofEGR measurement, drivability and fuel economy are improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of an example engine system includingan intake humidity sensor.

FIG. 2 shows a high level flow chart for disabling EGR in response to anexhaust air-fuel ratio becoming leaner than a threshold.

FIG. 3 shows an example disablement of EGR during selected leanoperating conditions.

DETAILED DESCRIPTION

Methods and systems are provided for disabling EGR during selected leanengine operating conditions in an engine system, such as the system ofFIG. 1. An engine controller may be configured to perform a controlroutine, such as the routine of FIG. 2, to disable EGR when enginecombustion is determined to be leaner than a threshold, where thethreshold is adjusted based on engine speed and load. An exampleadjustment is shown with reference to FIG. 3. In this way, misdiagnosisand miscalculation of EGR is reduced.

FIG. 1 shows a schematic depiction of an example turbocharged enginesystem 100 including a multi-cylinder internal combustion engine 10 andtwin turbochargers 120 and 130. As one non-limiting example, enginesystem 100 can be included as part of a propulsion system for apassenger vehicle. Engine system 100 can receive intake air via intakepassage 140. Intake passage 140 can include an air filter 156 and an EGRthrottle valve 230. Engine system 100 may be a split-engine systemwherein intake passage 140 is branched downstream of EGR throttle valve230 into first and second parallel intake passages, each including aturbocharger compressor. Specifically, at least a portion of intake airis directed to compressor 122 of turbocharger 120 via a first parallelintake passage 142 and at least another portion of the intake air isdirected to compressor 132 of turbocharger 130 via a second parallelintake passage 144 of the intake passage 140.

The first portion of the total intake air that is compressed bycompressor 122 may be supplied to intake manifold 160 via first parallelbranched intake passage 146. In this way, intake passages 142 and 146form a first parallel branch of the engine's air intake system.Similarly, a second portion of the total intake air can be compressedvia compressor 132 where it may be supplied to intake manifold 160 viasecond parallel branched intake passage 148. Thus, intake passages 144and 148 form a second parallel branch of the engine's air intake system.As shown in FIG. 1, intake air from intake passages 146 and 148 can berecombined via a common intake passage 149 before reaching intakemanifold 160, where the intake air may be provided to the engine.

A first EGR throttle valve 230 may be positioned in the engine intakeupstream of the first and second parallel intake passages 142 and 144,while a second air intake throttle valve 158 may be positioned in theengine intake downstream of the first and second parallel intakepassages 142 and 144, and downstream of the first and second parallelbranched intake passages 146 and 148, for example, in common intakepassage 149.

In some examples, intake manifold 160 may include an intake manifoldpressure sensor 182 for estimating a manifold pressure (MAP) and/or anintake manifold temperature sensor 183 for estimating a manifold airtemperature (MCT), each communicating with controller 12. Intake passage149 can include a charge air cooler (CAC) 154 and/or a throttle (such assecond throttle valve 158). The position of throttle valve 158 can beadjusted by the control system via a throttle actuator (not shown)communicatively coupled to controller 12. An anti-surge valve 152 may beprovided to selectively recirculate flow through the compressor stagesof turbochargers 120 and 130 via recirculation passage 150. As oneexample, anti-surge valve 152 can open to enable flow throughrecirculation passage 150 when the intake air pressure upstream of thecompressors attains a threshold value.

Air duct 149 may further include an intake gas oxygen sensor 172. In oneexample, the oxygen sensor is a UEGO sensor. As elaborated herein, theintake gas oxygen sensor may be configured to provide an estimateregarding the oxygen content of fresh air received in the intakemanifold. In addition, when EGR is flowing, a change in oxygenconcentration at the sensor may be used to infer an EGR amount and usedfor accurate EGR flow control. In the depicted example, oxygen sensor172 is positioned upstream of throttle 158 and downstream of charge aircooler 154. However, in alternate embodiments, the oxygen sensor may bepositioned upstream of the CAC. A pressure sensor 174 may be positionedalongside the oxygen sensor for estimating an intake pressure at whichan output of the oxygen sensor is received. Since the output of theoxygen sensor is influenced by the intake pressure, a reference oxygensensor output may be learned at a reference intake pressure. In oneexample, the reference intake pressure is a throttle inlet pressure(TIP) where pressure sensor 174 is a TIP sensor. In alternate examples,the reference intake pressure is a manifold pressure (MAP) as sensed byMAP sensor 182.

Engine 10 may include a plurality of cylinders 14. In the depictedexample, engine 10 includes six cylinders arrange in a V-configuration.Specifically, the six cylinders are arranged on two banks 13 and 15,with each bank including three cylinders. In alternate examples, engine10 can include two or more cylinders such as 3, 4, 5, 8, 10 or morecylinders. These various cylinders can be equally divided and arrangedin alternate configurations, such as V, in-line, boxed, etc. Eachcylinder 14 may be configured with a fuel injector 166. In the depictedexample, fuel injector 166 is a direct in-cylinder injector. However, inother examples, fuel injector 166 can be configured as a port based fuelinjector.

Intake air supplied to each cylinder 14 (herein, also referred to ascombustion chamber 14) via common intake passage 149 may be used forfuel combustion and products of combustion may then be exhausted fromvia bank-specific parallel exhaust passages. In the depicted example, afirst bank 13 of cylinders of engine 10 can exhaust products ofcombustion via a first parallel exhaust passage 17 and a second bank 15of cylinders can exhaust products of combustion via a second parallelexhaust passage 19. Each of the first and second parallel exhaustpassages 17 and 19 may further include a turbocharger turbine.Specifically, products of combustion that are exhausted via exhaustpassage 17 can be directed through exhaust turbine 124 of turbocharger120, which in turn can provide mechanical work to compressor 122 viashaft 126 in order to provide compression to the intake air.Alternatively, some of the exhaust gases flowing through exhaust passage17 can bypass turbine 124 via turbine bypass passage 123 as controlledby wastegate 128. Similarly, products of combustion that are exhaustedvia exhaust passage 19 can be directed through exhaust turbine 134 ofturbocharger 130, which in turn can provide mechanical work tocompressor 132 via shaft 136 in order to provide compression to intakeair flowing through the second branch of the engine's intake system.Alternatively, some of the exhaust gas flowing through exhaust passage19 can bypass turbine 134 via turbine bypass passage 133 as controlledby wastegate 138.

In some examples, exhaust turbines 124 and 134 may be configured asvariable geometry turbines, wherein controller 12 may adjust theposition of the turbine impeller blades (or vanes) to vary the level ofenergy that is obtained from the exhaust gas flow and imparted to theirrespective compressor. Alternatively, exhaust turbines 124 and 134 maybe configured as variable nozzle turbines, wherein controller 12 mayadjust the position of the turbine nozzle to vary the level of energythat is obtained from the exhaust gas flow and imparted to theirrespective compressor. For example, the control system can be configuredto independently vary the vane or nozzle position of the exhaust gasturbines 124 and 134 via respective actuators.

Exhaust gases in first parallel exhaust passage 17 may be directed tothe atmosphere via branched parallel exhaust passage 170 while exhaustgases in second parallel exhaust passage 19 may be directed to theatmosphere via branched parallel exhaust passage 180. Exhaust passages170 and 180 may include one or more exhaust after-treatment devices 176,such as a catalyst, and one or more exhaust gas sensors.

Engine 10 may further include one or more exhaust gas recirculation(EGR) passages, or loops, for recirculating at least a portion ofexhaust gas from the exhaust manifold to the intake manifold. These mayinclude high-pressure EGR loops for proving high-pressure EGR (HP-EGR)and low-pressure EGR-loops for providing low-pressure EGR (LP-EGR). Inone example, HP-EGR may be provided in the absence of boost provided byturbochargers 120, 130, while LP-EGR may be provided in the presence ofturbocharger boost and/or when exhaust gas temperature is above athreshold. In still other examples, both HP-EGR and LP-EGR may beprovided simultaneously.

In the depicted example, engine 10 may include a low-pressure EGR loop202 for recirculating at least some exhaust gas from the first branchedparallel exhaust passage 170, downstream of the turbine 124, to thefirst parallel intake passage 142, upstream of the compressor 122. Insome embodiments, a second low-pressure EGR loop (not shown) may belikewise provided for recirculating at least some exhaust gas from thesecond branched parallel exhaust passage 180, downstream of the turbine134, to the second parallel intake passage 144, upstream of thecompressor 132. LP-EGR loop 202 may include LP-EGR valve 204 forcontrolling an EGR flow (i.e., an amount of exhaust gas recirculated)through the loops, as well as an EGR cooler 206 for lowering atemperature of exhaust gas flowing through the EGR loop beforerecirculation into the engine intake. Under certain conditions, the EGRcooler 206 may also be used to heat the exhaust gas flowing throughLP-EGR loop 202 before the exhaust gas enters the compressor to avoidwater droplets impinging on the compressors.

In some examples, an EGR monitor 205 may be coupled to the low pressureEGR system, specifically, at the LP-EGR valve 204. In one example, EGRmonitor 205 may measure a delta pressure across the LP-EGR valve toinfer an EGR flow. The monitor may then diagnose the LP-EGR system basedon discrepancies between the EGR flow expected based on the deltapressure and an estimated EGR flow, such as based on intake oxygensensor 172. EGR monitor may be coupled to controller 12 and may includea counter. A count of the EGR monitor may be incremented in response toan EGR system error. When the count exceeds a threshold, a diagnosticcode may be set. Alternatively, the EGR monitor may include reading adelta pressure over an orifice separate from the EGR valve or an EGRhot-wire or hot-film anemometer mass flow meter.

Engine 10 may further include a first high-pressure EGR loop 208 forrecirculating at least some exhaust gas from the first parallel exhaustpassage 17, upstream of the turbine 124, to the first branched parallelintake passage 146, downstream of the compressor 122. Likewise, theengine may include a second high-pressure EGR loop (not shown) forrecirculating at least some exhaust gas from the second parallel exhaustpassage 18, upstream of the turbine 134, to the second branched parallelintake passage 148, downstream of the compressor 132. EGR flow throughHP-EGR loops 208 may be controlled via HP-EGR valve 210. As such, HP-EGRmay be injected downstream of the engine throttle 150 to improve theflow capability under some operating conditions.

A PCV port 102 may be configured to deliver crankcase ventilation gases(blow-by gases) to the engine intake manifold along second parallelintake passage 144. In some embodiments, flow of PCV air through PCVport 102 (e.g., PCV flow) may be controlled by a dedicated PCV portvalve. Likewise, a purge port 104 may be configured to deliver purgegases from a fuel system canister to the engine intake manifold alongpassage 144. In some embodiments, flow of purge air through purge port104 may be controlled by a dedicated purge port valve.

Humidity sensor 232 and pressure sensor 234 may be included in only oneof the parallel intake passages (herein, depicted in the first parallelintake air passage 142 but not in the second parallel intake passage144), downstream of EGR throttle valve 230. Specifically, the humiditysensor and the pressure sensor may be included in the intake passage notreceiving the PCV or purge air. Humidity sensor 232 may be configured toestimate a relative humidity of the intake air. In one embodiment,humidity sensor 232 is a UEGO sensor configured to estimate the relativehumidity of the intake air based on the output of the sensor at one ormore voltages. Since purge air and PCV air can confound the results ofthe humidity sensor, the purge port and PCV port are positioned in adistinct intake passage from the humidity sensor. Alternatively, theymay be positioned downstream of the humidity sensor. Pressure sensor 234may be configured to estimate a pressure of the intake air. In someembodiments, a temperature sensor may also be included in the sameparallel intake passage, downstream or upstream of the EGR throttlevalve 230.

As such, intake oxygen sensor 172 may be used for estimating an intakeoxygen concentration and inferring an amount of EGR flow through theengine based on a change in the intake oxygen concentration upon openingof the EGR valve 204. Specifically, a change in the output of the sensorupon opening the EGR valve is compared to a reference point where thesensor is operating with no EGR (the zero point). Based on the change(e.g., decrease) in oxygen amount from the time of operating with noEGR, an EGR flow currently provided to the engine can be calculated. Forexample, upon applying a reference voltage (Vs) to the sensor, a pumpingcurrent (Ip) is output by the sensor. The change in oxygen concentrationmay be proportional to the change in pumping current (delta Ip) outputby the sensor in the presence of EGR relative to sensor output in theabsence of EGR (the zero point). Based on a deviation of the estimatedEGR flow from the expected (or target) EGR flow, further EGR control maybe performed.

In one example, a zero point estimation of the intake oxygen sensor maybe performed during idle conditions where intake pressure fluctuationsare minimal and when no PCV or purge air is ingested into the engine. Inaddition, the idle adaptation may be performed periodically, such as atevery first idle following an engine start, to compensate for the effectof sensor aging and part-to-part variability on the sensor output.

A zero point estimation of the intake oxygen sensor may alternatively beperformed during engine non-fueling conditions, such as during adeceleration fuel shut off (DFSO). By performing the adaptation duringDFSO conditions, in addition to reduced noise factors such as thoseachieved during idle adaptation, sensor reading variations due to EGRvalve leakage can be reduced.

Returning to FIG. 1, the position of intake and exhaust valves of eachcylinder 14 may be regulated via hydraulically actuated lifters coupledto valve pushrods, or via a cam profile switching mechanism in which camlobes are used. Specifically, the intake valve cam actuation system 25may include one or more cams and may utilize variable cam timing or liftfor intake and/or exhaust valves. In alternative embodiments, the intakevalves may be controlled by electric valve actuation. Similarly, theexhaust valves may be controlled by cam actuation systems or electricvalve actuation.

Engine system 100 may be controlled at least partially by a controlsystem 15 including controller 12 and by input from a vehicle operatorvia an input device (not shown). Control system 15 is shown receivinginformation from a plurality of sensors 16 (various examples of whichare described herein) and sending control signals to a plurality ofactuators 81. As one example, sensors 16 may include humidity sensor232, intake air pressure sensor 234, MAP sensor 182, MCT sensor 183, TIPsensor 174, EGR monitor 205, and intake air oxygen sensor 172. In someexamples, common intake passage 149 may further include a throttle inlettemperature sensor for estimating a throttle air temperature (TCT). Inother examples, one or more of the EGR passages may include pressure,temperature, and hot-wire or hot-film anemometer flow sensors, fordetermining EGR flow characteristics. As another example, actuators 81may include fuel injector 166, HP-EGR valve 210, LP-EGR valve 204,throttle valves 158 and 230, and wastegates 128, 138. Other actuators,such as a variety of additional valves and throttles, may be coupled tovarious locations in engine system 100. Controller 12 may receive inputdata from the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines. Anexample control routine is described herein with regard to FIG. 2.

Now turning to FIG. 2, an example routine 200 is shown for disabling EGRduring selected lean engine operating conditions. The method allowsissues associated with the misdiagnosis of an EGR system as well as EGRestimation errors to be reduced.

At 252, the routine includes estimating and/or measuring engineoperating conditions. These may include, for example, engine speed,load, barometric pressure, driver torque demand, engine temperature, NOxlevels, air-fuel ratio, etc. At 254, based on the estimated operatingconditions, a target EGR may be determined. This may include, forexample, determining an EGR flow rate, amount, temperature, etc.Further, this may include determining an amount of high pressure EGR,low pressure EGR or a ratio thereof. Accordingly, an EGR valve positionmay be adjusted to provide the target EGR.

In one example, EGR may be flowing while the engine is operating with anair-fuel ratio adjusted to be leaner than stoichiometry. The lean engineoperation may be due to the engine speed-load conditions to reducepumping losses or during deceleration fuel shutoff (DFSO). Othercircumstances can create extra air in the exhaust such as blow through.As such, the engine may be operating boosted and fueled via directinjection. The air-fuel ratio adjusted to be leaner than stoichiometrymay be a combustion air and fuel combusting in a cylinder of the engine,the combustion initiated via spark-ignition, resulting in excess air inthe exhaust and hence, in the EGR.

At 258, a lean threshold may be determined based on the exhaust UEGOsensor reading. The lean threshold may reflect an upper limit of leanoperation. Beyond the lean threshold, EGR estimation and diagnosiserrors may occur. Specifically, if the engine is operated at an exhaustair-fuel ratio leaner than the lean threshold, the excess oxygen presentin the exhaust may cause false readings of EGR measurement and EGRsystem errors. For example, if the exhaust includes an amount of freshair, the EGR will also include fresh air from the exhaust side. Inengine systems where an EGR monitor measures or monitors the EGR flow bymeasuring a delta pressure across the valve or across a fixed orifice orEGR MAF sensor, the additional fresh air may be interpreted as EGR.Based on this information, spark timing, throttle opening, and otheractuators may be scheduled incorrectly leading to combustion and/ortorque control issues. In engine systems where the EGR concentration ismeasured by an intake oxygen sensor, the fresh air may lead to an EGRmeasurement error. The error may cause the oxygen sensor output to beinterpreted as a drop in dilution, or as a low EGR flowing condition.This in turn can cause diagnostic issues due to a discrepancy betweenthe expected EGR valve flow versus the EGR measured by the intake oxygensensor. In MAF systems, the fresh air through the EGR system may beunaccounted for, leading to air charge calculation errors. These, inturn, may lead to fueling and torque errors.

To reduce these errors, as elaborated below, if the lean engineoperation exceeds a lean threshold, EGR may be disabled. In other words,during conditions where the exhaust oxygen concentration exceeds, or isexpected to exceed, the calibratable lean threshold, EGR may bedisabled. As such, the lean threshold may be determined by the maximumallowable EGR measurement error and/or EGR flow diagnostic detectionrequirement. In one example the flow based monitor may over-estimate thedilution and lead to a modification of spark timing or throttle anglethat reduces fuel efficiency, degrades combustion stability or increasesemissions. In another example, the difference between the flow basedmonitor and the EGR dilution measurement may exceed a threshold and beincorrectly interpreted as a sensor fault or other EGR system fault. Asone example, adjusting or calibrating the threshold (based on the enginespeed/load) includes estimating an EGR error upper limit that can betolerated at the current engine speed/load, estimating an air errorupper limit based on the EGR error upper limit, and calculating thethreshold based on the air error upper limit. Thus, the threshold may beincreased as the engine speed/load increases.

At 260, the routine includes estimating an exhaust air-fuel ratio. Forexample, the exhaust (or combustion) air-fuel ratio may be estimated bythe exhaust oxygen sensor. As previously discussed, the intake oxygensensor may be configured to estimate an EGR flow to the engine based ona change in oxygen concentration. The exhaust oxygen sensor ispositioned upstream of an exhaust emission control device of the exhaustmanifold (e.g., an exhaust three-way catalyst).

At 262, it may be determined if the estimated exhaust air-fuel ratio isleaner than the threshold. If not, then at 264, EGR valve control may beadjusted to provide the target EGR. Herein, EGR valve opening/closing isadjusted based on differences between the estimated EGR and the targetEGR. This includes increasing an opening of the EGR valve to increaseEGR delivery if the estimated EGR rate is lower than the target EGR rateand decreasing the opening of the EGR valve to decrease EGR delivery ifthe estimated EGR rate is higher than the target EGR rate. In additionto EGR control, spark control and torque control based on EGR may bemaintained.

In comparison, in response to the exhaust air-fuel ratio being leanerthan the threshold while operating the engine with an air-fuel ratiothat is leaner than stoichiometry, at 266, the routine includes closingthe EGR valve. Closing the EGR valve includes fully closing the EGRvalve and fully sealing the EGR valve against its seat. As such, the EGRvalve may be coupled to a low pressure EGR passage such that by closingthe EGR valve, low pressure EGR may be disabled. Closing the EGR valvefurther includes fully closing the EGR valve irrespective of adifference between EGR estimated (e.g., by the intake oxygen sensor) anda target EGR (e.g., based on the engine speed and load).

Next, at 270, the routine includes adjusting a valve timing to increaseinternal EGR, for example to compensate for the drop in external EGR.Adjusting the valve timing includes adjusting one or more of an intakevalve timing, an exhaust valve timing, and an amount of valve overlap.For example, the adjusting may include advancing intake cam timingand/or retarding exhaust valve timing to increase valve overlap,replacing external EGR with internal EGR, or otherwise changing campositions to improve fuel economy and/or emissions.

At 272, spark control and torque control of the engine may be adjustedto account for higher levels of internal EGR and the cessation ofexternal EGR. For example, the spark may be retarded if the adjusted camposition results in less total residual dilution. The throttle may beadjusted to provide the requested torque at the adjusted cam positionfrom step 270. In some examples, the EGR may be maintained closed untilthe exhaust air-fuel ratio is outside the lean threshold. For example,the EGR valve may be opened when the air-fuel ratio is at or below thethreshold. In one example, the EGR valve may be opened responsive tostoichiometric engine operation.

In one example, an engine method comprises flowing low pressure EGR; andin response to an exhaust air-fuel ratio becoming leaner than athreshold based on engine speed and load, disabling the EGR. Herein,each of the flow rate of the flowing EGR and the exhaust air-fuel ratioare estimated by intake and exhaust oxygen sensors, respectively. Themethod further comprises, while flowing low pressure EGR, incrementing acounter of an EGR monitor responsive to an indication of EGR error, andwhile disabling the EGR, resetting or suspending the counter of the EGRmonitor.

In another example, an engine system comprises an engine with an intakeand an exhaust; an EGR system including an EGR passage for recirculatingexhaust residuals from the exhaust to the intake via an EGR valve; andan EGR monitor including a counter. The engine system further includesan intake oxygen sensor coupled to the intake, upstream of an intakethrottle and downstream of an outlet of the EGR passage, the sensorconfigured to estimate EGR. An engine controller may be configured withcomputer readable instructions for: flowing EGR based on engine speedand load and during the flowing, incrementing the counter in response toan indication of EGR system error. Then, in response to an exhaustair-fuel ratio rising above a lean threshold, the controller may closethe EGR valve. Herein, closing the EGR valve includes fully closing theEGR valve irrespective of a difference between EGR estimated by theintake oxygen sensor and a target EGR based on the engine speed andload.

It will be appreciated that while the routine of FIG. 2 shows disablingEGR during conditions when engine operation is too lean, EGR may besimilarly disabled during other conditions where the exhaust has toomuch fresh air, such as when fuel injectors are disabled and the exhaustoxygen concentration is higher than a calibrated threshold. This mayinclude, for example, deceleration fuel shut-off conditions, as well asindividual cylinder fuel shut-off conditions.

Now turning to FIG. 3, an example EGR disablement during selected enginelean conditions is shown. Map 300 depicts changes in an exhaust air-fuelratio (AFR) at plot 302, changes in the output of an intake oxygensensor at plot 304, estimated EGR (based on the output of the intakeoxygen sensor) at plot 306, actual EGR at plot 308, an EGR enablementflag at plot 310, and an EGR monitor counter at plot 312.

Prior to t1, the engine may be operating at or around stoichiometry(plot 302). Between t0 and t1, the EGR being delivered to the engine maybe estimated based on the output of the intake oxygen sensor. Forexample, in response to a drop in oxygen concentration (half way betweent0 and t1), a rise in engine dilution from the delivery of residuals maybe inferred and the estimated EGR may be accordingly increased (compareplot 304 and 306). Herein, the estimated EGR may correctly reflect theactual EGR (plot 308).

Also between t0 and t1, an EGR monitor may be monitoring the EGR system.Specifically, the EGR monitor may monitor changes in the EGR valveposition and the corresponding changes in EGR delivery (e.g., based onEGR amount, differential pressure across the EGR valve, etc.) and mayindicate any errors by incrementing a counter. In the present example,between t0 and t1, there may be a single EGR error that is identified bythe EGR monitor and the counter may be accordingly incremented. However,due to the counter reading being lower than a threshold 311, no EGRdegradation code may have been set.

At t1, lean engine operation may be requested and accordingly, theengine air-fuel ratio may be enleaned. Between t1 and t2, the engineair-fuel ratio may be operated leaner than stoichiometry, with a gradualenleanment of the air-fuel ratio. Herein, between t1 and t2, there maybe a single EGR error that is identified by the EGR monitor and thecounter may be accordingly incremented. However, due to the counterreading being lower than threshold 311, no EGR degradation code may havebeen set.

At t2, the combustion or exhaust air-fuel ratio may reach a leanthreshold 303. As such, engine operation at an air-fuel ratio leanerthan lean threshold 303 may lead to errors in EGR estimation anddelivery. Specifically, due to the enleanment, there may be asignificant amount of fresh air in the exhaust and therefore in the EGR.As a result, the intake oxygen sensor may read a gradually increasingoxygen concentration and accordingly infer a gradually decreasing enginedilution. Consequently, the EGR estimated based on the intake oxygensensor output may be lower than the actual EGR delivered. EGR deliverycontrol based on the incorrect EGR estimate may lead to incorrect sparkand torque control, degrading engine performance. In addition, the EGRmonitor may incorrectly diagnose EGR system errors more frequently andincrement the counter faster.

To avoid these issues, at t2, in response to the exhaust air-fuel ratiobeing leaner than threshold 303, EGR may be disabled. For example, anEGR valve may be closed. As used herein, the EGR refers to low pressureEGR and the EGR valve refers to an EGR valve coupled in a low pressureEGR passage. As a result of the EGR disablement, incorrect EGRestimation is stopped. The controller may also set a flag at 310 toindicate that EGR has been disabled due to the presence of too muchfresh air in the exhaust. In response to the flag being set, spark andtorque control may be adjusted. In addition, the counter may besuspended such that the counter shows the same reading before and aftert2.

In this way, issues associated with incorrect EGR estimation arereduced. Specifically, spark and torque control is improve, whichimproves engine drivability and fuel economy. In addition, issuesassociated with the misdiagnosis of an EGR system by an EGR monitor canbe reduced. Specifically, an EGR system may not be flagged as degradedwhen it is actually functional. As such, this reduces costs associatedwith failed EGR monitors. By disabling EGR during conditions when falsereading of EGR measurement is possible, vehicle performance is improved.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations,and/or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedactions, operations and/or functions may be repeatedly performeddepending on the particular strategy being used. Further, the describedactions, operations and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the engine control system.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-3, I-4, I-6, V-12, opposed 4, and other engine types. The subjectmatter of the present disclosure includes all novel and non-obviouscombinations and sub-combinations of the various systems andconfigurations, and other features, functions, and/or propertiesdisclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

The invention claimed is:
 1. A method, comprising: while operating anengine with an air-fuel ratio adjusted to be leaner than stoichiometry,in response to exhaust air-fuel ratio being leaner than a threshold,closing an EGR valve, the threshold increased as an engine speed andload increases.
 2. The method of claim 1, wherein the exhaust air-fuelratio is measured by an exhaust oxygen sensor.
 3. The method of claim 1,wherein the threshold is based on engine speed and load.
 4. The methodof claim 1, further comprising opening the EGR valve when the exhaustair-fuel ratio is at or below the threshold.
 5. The method of claim 1,further comprising, in response to the exhaust air-fuel ratio beingleaner than the threshold, suspending an EGR monitor.
 6. The method ofclaim 1, wherein the engine is boosted, fueled via direct injection, andfurther comprises an exhaust manifold which includes a three-waycatalyst.
 7. The method of claim 6, wherein the exhaust air-fuel ratiois estimated by an exhaust oxygen sensor positioned upstream of thethree-way catalyst.
 8. The method of claim 1, further comprising, afterclosing the EGR valve, adjusting a valve timing to increase internalEGR.
 9. The method of claim 8, wherein adjusting the valve timingincludes one or more of advancing the timing of intake valve opening orretarding the timing of exhaust valve closing to increase valve overlap.10. The method of claim 1, wherein the air-fuel ratio adjusted to beleaner than stoichiometry is a combustion air-fuel ratio of air and fuelcombusting in a cylinder of the engine, the combustion initiated viaspark-ignition.
 11. The method of claim 10, wherein closing the EGRvalve includes fully sealing the EGR valve against a seat of the EGRvalve.
 12. The method of claim 11, wherein the EGR valve is coupled to alow pressure EGR passage and wherein closing the EGR valve includesdisabling low pressure EGR.
 13. A method for an engine, comprising:flowing low pressure EGR; and in response to an exhaust air-fuel ratiobecoming leaner than a threshold based on engine speed and load,disabling the EGR; and while disabling the EGR, suspending a counter ofan EGR monitor.
 14. The method of claim 13, further comprising,estimating a flow rate of the flowing EGR via an intake oxygen sensorand measuring an exhaust oxygen content of the flowing EGR via anexhaust oxygen sensor.
 15. The method of claim 13, wherein the thresholdbeing based on the engine speed and load includes: estimating an EGRerror upper limit that can be tolerated at the engine speed and load;estimating an air error upper limit based on the EGR error upper limit;and calculating the threshold based on the air error upper limit. 16.The method of claim 13, further comprising, while flowing low pressureEGR, incrementing the counter of the EGR monitor responsive to anindication of EGR error.
 17. An engine system, comprising: an enginewith an intake and an exhaust; an EGR system including an EGR passagefor recirculating exhaust residuals from the exhaust to the intake viaan EGR valve; an EGR monitor including a counter; an intake oxygensensor coupled to the intake, upstream of an intake throttle anddownstream of an outlet of the EGR passage, the intake oxygen sensorconfigured to estimate one or more of a combustion air-fuel ratio andEGR; an exhaust oxygen sensor coupled to the exhaust, upstream of anemission control device, the exhaust oxygen sensor configured toestimate an exhaust air-fuel ratio; and a controller with computerreadable instructions for: flowing EGR based on engine speed and load;during the flowing, incrementing the counter in response to anindication of EGR system error; and in response to the exhaust air-fuelratio rising above a lean threshold, closing the EGR valve; andsuspending the counter.
 18. The system of claim 17, wherein closing theEGR valve includes fully closing the EGR valve irrespective of adifference between EGR estimated by the intake oxygen sensor and atarget EGR based on the engine speed and load.
 19. The system of claim18, wherein the controller includes further instructions for: afterclosing the EGR valve, adjusting engine cylinder valve timing toincrease internal EGR, the adjusting cylinder valve timing including oneor more of advancing timing of an intake valve opening and retardingtiming of an exhaust valve closing to increase valve overlap.